The present invention relates to digital television systems, such as the digital high-definition television (HDTV) system used for terrestrial broadcasting in the United States of America in accordance with the Advanced Television Systems Committee (ATSC) standard, and more particularly, to digital television (DTV) receivers with circuitry for suppressing artifacts of co-channel interference from analog television signals, such as those conforming to the National Television System Committee (NTSC) standard.
The carrier of a vestigial-sideband (VSB) digital television (DTV) signal broadcast in accordance with the Digital Television Standard published Sep. 16, 1995 by the ATSC is nominally 310 KHz above the lower limit frequency television channel assignment. The artifacts of the co-channel interference in the demodulated DTV signal include beats at about 56.86 times the horizontal scan line frequency fH of the NTSC analog TV signal, generated by heterodyne between the digital HDTV carrier and the video carrier of the co-channel interfering analog TV signal. The artifacts include beats at about 284.36 times fH, generated by heterodyne between the digital HDTV carrier and the chrominance subcarrier of the co-channel interfering analog TV signal, which beats are quite close in frequency to the fifth harmonic of the beats at 56.86 times fH. The artifacts will further include beats at approximately 342.86 times fH, generated by heterodyne between the digital HDTV carrier and the audio carrier of the co-channel interfering analog TV signal, which beats are quite close in frequency to the sixth harmonic of the beats at 56.86 times fH. The nearly harmonic relationship of these beats allows them all to be suppressed by a single properly designed comb filter incorporating twelve symbol epochs of differential delay. The use of an NTSC-rejection comb filter before data-slicing in the DTV receiver incidentally introduces intersymbol interference.
U.S. Pat. No. 5,132,797 issued Jul. 21, 1992 to R. W. Citta and entitled xe2x80x9cCO-CHANNEL INTERFERENCE FILTER FOR DIGITAL HIGH DEFINITION TELEVISION RECEIVERxe2x80x9d describes filtering for reducing artifacts of co-channel interfering NTSC analog television signals in digital television signals converted to baseband. So does U.S. Pat. No. 5,162,900 issued Nov. 10, 1992 to R. W. Citta and entitled xe2x80x9cCO-CHANNEL INTERFERENCE FILTER FOR TELEVISION RECEIVERxe2x80x9d. A comb filter for rejecting the artifacts in baseband symbol coding generates a comb filter response subject to intersymbol distortion. The comb filter response is supplied to an intersymbol interference filter, the response of which provides baseband symbol coding without intersymbol distortion and with reduced NTSC artifacts to the data slicer used to implement symbol decoding. Data slicing results from the data slicer are fed back to the intersymbol interference filter for linear combination with the comb filter response to remove intersymbol distortion before data slicing. This allows the use of a data slicer having 2N amplitude bins or slicing ranges, 2N being the number of levels in baseband symbol coding before comb filtering which generates a (4Nxe2x88x921)-level response. N is a positive integer having a value of four in over-the-air signals transmitted in accordance with the ATSC Digital Television Standard and having a value of eight in cablecast signals transmitted in accordance with that standard.
The Citta apparatus not only requires a delay element in the comb filter, but also requires a corresponding delay element in the intersymbol interference filter. When the delay required of each of these delay elements is only a few symbols, this is not too serious a shortcoming. But comb filtering over several NTSC scan lines is doubly expensive in the costs of delay elements.
U.S. Pat. No. 5,748,266 issued May 5, 1998 to A. L. R. Limberg and entitled xe2x80x9cDIGITAL TELEVISION RECEIVER WITH ADAPTIVE FILTER CIRCUITRY FOR SUPPRESSING NTSC CO-CHANNEL INTERFERENCExe2x80x9d describes the response of a comb filter for rejecting the artifacts in baseband symbol coding being supplied directly to a data slicer, with intersymbol interference being corrected for in the data slicing results. A data slicer having (4Nxe2x88x921) amplitude bins or slicing ranges is required when intersymbol interference is corrected for in the data slicing results. The range of amplitude in each of the amplitude bins or slicing ranges of the data slicer is halved compared with the range of amplitude in each of the amplitude bins or slicing ranges of a data slicer used for symbol decoding baseband symbol coding that has not been comb filtered. This reduction in the range of amplitude in each of the amplitude bins or slicing ranges makes error in data slicing owing to NTSC artifacts or noise more likely to occur than in a scheme in which intersymbol interference is corrected for before the data slicer.
An objective of the invention is to supply the data slicer with baseband symbol coding in which artifacts of co-channel interfering NTSC analog television signals and intersymbol distortion are suppressed. At the same time a reduction in the number of digital delay elements required in the Citta apparatus is sought.
The invention is embodied in a digital television signal receiver including demodulation circuitry for supplying digital samples of baseband symbol code, which samples are accompanied at times by unwanted artifacts of co-channel analog television signal, and including symbol decoding circuitry of the following sort. A first linear combiner combines digital samples of a predicted estimate of the unwanted artifacts of co-channel analog television signal with digital samples of the baseband symbol code supplied from the demodulation circuitry, to supply digital samples of filtered baseband symbol code with reduction of unwanted artifacts of co-channel analog television signal. A data slicer responsive to each symbol in the filtered baseband symbol code supplies a respective group of bits as data slicing results. A read-only memory addressed by each successive group of bits supplied as data slicing results from the data slicer supplies a digital sample indicative of the center of the data slice from which that respective group of bits originates. A second linear combiner combines each successive digital sample of the baseband symbol code from the demodulation circuitry with the digital sample indicative of the center of the data slice, for generating a digital sample descriptive of a revised estimate of the unwanted artifacts of co-channel analog television signal for a respective later symbol. A delay line is used for delaying the revised estimates of the unwanted artifacts of co-channel analog television signal to be supplied to the first linear combiner as predicted estimates of the unwanted artifacts of co-channel analog television signal that will accompany respective later symbols. Symbol decoding results are supplied in response to the groups of bits the data slicer supplies as its data slicing results.